The present invention relates generally to airborne navigational systems and, more particularly, to devices and methods to determine the location of an airborne vehicle relative to a target area. Most particularly, the present invention relates to a system for determining the location between an airborne vehicle relative to a target area which operates without the need for aiding from the Global Positioning System (GPS) and is operable in all-weather.
Conventionally, various approaches exist to determine the three-dimensional geolocation of an airborne vehicle (for example, an aircraft or a missile) relative to a particular target. The target may be an airport, landing strip, bombing target, power plant or other high value fixed targets. Most conventional approaches operate with the aid of an inertial navigation system (INS) or use various aiding devices, sensors, and receivers, such as GPS receivers. These existing systems provide updates to navigational systems and terminal guidance and rely heavily on information obtained through the aiding devices. As is known, the operational capability of these aiding devices degrades with adverse weather conditions. Also, GPS signals are vulnerable to jamming.
To date, the only known devices that meet the requirements of precision, all-weather, GPS-independent guidance are synthetic aperture radars (SAR). Most existing SARs are two-dimensional, forward-looking devices (with a squint angle), which do not extract elevation data and do not have a downward-looking mode. As an example, one SAR is known as the HAMMERHEAD radar system, which is a radar system being developed by Raytheon Corporation. The slant range position of a particular target location can be determined by measuring the time delay between transmitting and receiving of an electromagnetic pulse and the Doppler frequency of the returns. Digital signal processing techniques may then be used to produce high-resolution, two-dimensional image maps of the target area.
An improved SAR, known as an interferometric SAR (IFSAR), uses two spatially displaced antennas. The dual antenna interferometric SAR is capable of obtaining information related to altitude, thus producing three-dimensional topography maps of the target area as well as an associated, co-registered gray-scale intensity image. Such three-dimensional information is obtained using the known location of the airborne vehicle hosting the interferometric SAR obtained from an independent source such as GPS units. An improved position locating device is described in U.S. Pat. No. 5,867,119, incorporated herein in its entirety for all purposes, which can be used to provide automatic position location for a moving airborne vehicle by correlating a sensed array of altitudes with a stored terrain reference. This improved system eliminates the need for an independent location reference.
There is, however, a continuing need to determine the location of an airborne platform relative to a target area. A preferred system would determine such location based on both relative terrain height information and gray-level imagery. The system would provide precision guidance in all weather without the aid of GPS information and would preferably integrate the features of midcourse navigation update and terminal guidance into a single system. The preferred system would reside on a moving airborne vehicle or platform and would provide the location of a target or target area relative to the airborne vehicle.
The present invention provides an integrated system for determining the location of a target area relative to an airborne vehicle. The invention incorporates an interferometric synthetic aperture radar to sense both three-dimensional location information and two-dimensional gray scale intensity information relative to the target area and compares such information with previously-stored references of similar information to locate the target area. The comparison results in a geometric location of the airborne vehicle and assists the airborne vehicle in guidance toward the target area.
In a preferred embodiment, the present invention includes an apparatus to determine the three-dimensional location of an airborne platform relative to a target area using two separate IFSAR antenna assemblies positioned at the airborne platform. The first antenna assembly is adapted to transmit energy downward toward a surface location directly beneath the airborne platform, while the second antenna assembly is adapted to transmit energy forward towards the target area with a squint angle in azimuth and elevation relative to the velocity vector. A single transmitter is shared between the two antenna assemblies for transmitting signals toward the surface location and the target area at non-overlapping intervals. A single receiver coupled to the antenna assemblies receives and detects the signals corresponding to the transmitted energy as reflected by the target area and the surface location. A radar processor is coupled to the receiver and is adapted to determine the range between the airborne platform and the surface location and determine the three-dimensional height of the target area from the detected signals. The radar processor also extracts gray scale imagery of the target area from the detected signals. The apparatus also incorporates a set of geolocated stored references for each of the antenna assemblies. The reference sets consist of three-dimensional maps of terrain heights and their associated co-registered gray scale images of both the surface location and target area. A height correlator compares the elevation measurements produced by the platform sensors from both the downward and forward looking sensors to the stored reference of terrain heights. Also, an image correlator for comparing the extracted gray scale imagery of the target area to the stored reference of gray scale imagery is provided. A navigational processor then determines the three-dimensional location of the platform relative to the target area based on the correlation results of the height correlator and the image correlator.